A new expression cloning system for isolating tumor suppressor genes and growth inhibitory genes has been developed. The main components of this system are a regulated expression of cloned cDNA and episomal maintenance of transfected plasmid. The regulated expression system offers an essential tool for a transition between normal and tumorigenic phenotypes. The episomal replication system was achieved by using the replication system of Epstein-Barr virus. The transfection efficiency of human tumor cells was greatly increased using this system. Plasmid rescue from transfected human tumor cells will provide an advantage for analyzing positive clones.